The present invention relates to making an aluminum alloy product having improved combinations of strength, toughness and corrosion resistance, together with good formability in the form of a rolled sheet or strip product such that elongate sheet structural section airframe members, such as stringers and frames, can be roll formed from such strip.
Currently, aluminum alloy sheet or strip products are roll formed into elongate aerospace structural shape members, such as stringers, that run lengthwise along an airplane fuselage to reinforce the fuselage. A typical elongate stringer may have a hat-like cross-section shape achieved by the roll forming process. In some cases, the sheet or strip product is first taper rolled along its length to provide thinner portions at select locations along the length tapering back to the original thicker sheet or strip thickness. The tapering between thicker and thinner regions then again can occur several times along the length of the stringer. Typically, the thicker portions of the taper rolled strip can be aligned with fastener regions to provide the desired ruggedness and strength at the fastening site. After the taper rolling, the hat-like shape is roll formed from the multiple or dual thickness strip material to make the elongate stringer. In some cases, taper rolling is omitted. The roll forming operation is a serious formability operation when dealing with the high strength materials needed for aerospace application, especially if the taper rolling operation just referred to precedes roll forming.
The taper rolling operation itself can produce serious stresses in the material since the taper rolling can involve reductions of up to 50% to 70% or so in a single rolling pass which can result in serious degrees of edge cracking in the thinner rolled areas along the length. The edge cracking, as a minimum, creates more scrap by trimming or, more seriously, can result in scrapping an entire member.
The material presently used for many stringer applications in commercial airliner construction is 7075 alloy, usually processed to enhance formability, which after roll shaping is artificially aged to a T6 temper in which it has fairly high strength, which typically can be around 81 or 82 or 83 ksi ultimate longitudinal strength and around 73 or 74 ksi longitudinal tension yield strength so as to be useful in the application, but the 7075-T6 product is characterized by only moderate L-T fracture toughness of around 70 to 100 ksi.sqroot. in K.sub.c and by some susceptibility to stress corrosion cracking.
While the final temper of the product may be T6 temper, the forming operation is normally performed in an annealed temper, normally referred to as the "0" temper, or in a "W" temper which is solution heat treated, quenched and then refrigerated so as to reduce the natural aging strength increase and retain a suitable level of formability. Normally, the "0" temper is desired where taper rolling is employed so as to reduce edge cracking, but it is advantageous even if starting with the "0" temper to convert to the "W" temper by solution heat treating and quenching after taper rolling, if employed, and before roll shaping a stringer because a stringer made from roll formed "W" material will be less distorted than one made from "0" temper material and solution treated after shaping since rapid quenching after solution heating can cause significant amounts of distortion of the shaped stringer.
It is to be appreciated that, except as indicated otherwise herein, alloy designations (such as 7075) and temper designations (such as T6, 0, W) refer to the Aluminum Association designations in Aluminum Standards and Data and the Registration Records, all published by the Aluminum Association and all fully incorporated herein by reference.
As stated above, the present materials used in stringer and other roll formed applications leave considerable room for improvement, and it would be desirable to have a material which could substantially equal or possibly exceed the strength of 7075-T6 and combine that strength with a higher level of fracture toughness and corrosion resistance while not suffering from reduced formability as compared to 7075 alloy. For instance, 7075-T6 has typical stress corrosion cracking (SCC) resistance of 15 to possibly 25 ksi thresholds (no failures after 40 days' alternate immersion (AI) testing) at that stress level using ASTM G64, G44 test procedures. It would be desirable to achieve SCC thresholds of 30 to 35 ksi while not degrading other properties.